It is worth noting several limitations to the current study. The use of a treatment sample means firstly that the results may not be generalizable to those opioid users who have never been in treatment. All the same, there is evidence that most opioid dependent individuals in Australia have been in treatment at some point [1, 37]. Secondly, using a dependent sample means that the abuse and dependence data is restricted in range, which may be biased towards lesser order factor solutions [57]. The same analysis on a general population sample may yield different models. However because of the low population prevalence of opioid dependence, applying this analysis to a general population sample would require a larger sample size than is currently available. A further limitation is that the participants were predominantly heroin dependent, not opioid analgesic dependent. Differences between these two groups may mean that different subtypes and associations exist amongst opioid analgesic users [58]. Nevertheless, there may be significant overlap between the two groups, with almost half of IDU in one survey having injected morphine [59].
